stock jet size?

11657 Views 9 Replies 3 Participants Last post by PatC, Jan 23, 2015

kcducttaper

Discussion Starter · Jan 22, 2015

What is the stock jet size on the 1500a? My bike has intake and exhaust mods. I'm not sure if it has been re-jetted, but it was definitely popping quite a bit when I was decelerating. What's a typical jet size for "the big 3" mod?

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madhatter4119

· #2 · Jan 22, 2015 (Edited)

Here...use their calculator and it will recommend the jets to use...I buy my jets from
them...good prices and free shipping...good luck

http://carbjetkits.com/CJKCalculato...22&bname=Kawasaki VN1500A 1987-1998 Vulcan 88

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PatC

· #3 · Jan 22, 2015 (Edited)

The jets on your bike are a different size for each carb. In one carb they can be a Keihin size 118-122; in the other a keihin 110-115. Keep in mind Mikuni, Kehin and Dynojet all use a different numbering system, so you may order a 124 main from Dynojet and get a 132 Kehin, which would be quite a jump.

It may not be the jets at all. Decel popping is usually from the reed valves. If you marble it or put coaster covers on it the problem is likely to go away if it runs fine otherwise. http://www.ebay.com/itm/Kawasaki-Vulcan-1500A-Coaster-Reed-Valve-Cover-/321641936898

Tuning twin carb bikes with new jets is a nightmare.

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kcducttaper

Discussion Starter · #4 · Jan 22, 2015

Thanks for the info!

I figure I should at least pop the jets out to see what they are. If they're still the stockers, I probably ought to change them out. I'm not looking forward to tuning them though.

I'm not very familiar with these particular carbs yet, but don't they just have a HSN I can richen up a smidge to support the added air flow? Everyone one seems to be in favor of re-jetting rather than richening up the mixture a tad, but I can't seem to figure out why.

I also don't want to run the bike too lean for too long. That just messes with stuff. I'm not terribly familiar with moding the reed valves or marbling, but I'll look into that and see if it's already done or not. I thought reed valves were only used on 2-stroke engines and not 4-stroke.

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madhatter4119

· #5 · Jan 22, 2015

With your mods...blocking the reed valves is pretty much a MUST...There is plenty of free info on marbling...getting or making a
sync tool will be needed...Don't get up in your head about tuning your carb's...your spark
plugs will dictate the tuning...if the rubber (o-rings on the pilot screws/diaphragms ect) in your carb's are good...its doable...But this is all after you get that beast rebuilt...right?

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kcducttaper

Discussion Starter · #6 · Jan 22, 2015

Yeah, I'm gonna have to study up on marbling and such. One of my co-workers has a parallel 4 something or another with 4 carbs and has a sync tool, so I may have to borrow that for a bit.

I'm not that intimidated by tuning as I've tuned many carbs before. It just takes a while to get everything just so. As far as I have been able to tell, all of my seals and rubbers are still solid, but I haven't torn into the carb yet. I just figured if I needed to replace the jets, now would be the time to do it since everything is all strewn apart already.

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madhatter4119

· #7 · Jan 22, 2015

Just use the link in post #2...give you the jet size with your mods...

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PatC

· #8 · Jan 23, 2015

I'm not very familiar with these particular carbs yet, but don't they just have a HSN I can richen up a smidge to support the added air flow? Everyone one seems to be in favor of re-jetting rather than richening up the mixture a tad, but I can't seem to figure out why.

You can adjust the pilot screw to add more fuel, but the way a carb works is this" for the firs 1/4 throttle, it's the pilot its doing most of the work. Once you get 1/4 to 3/4 or so, it's the needle jet and needle. After 3/4 throttle, it's the main jet. Of course, they all overlap, but this is why simply adjusting the fuel/air mixture screw won't change much at fuller throttle. Some good info here:

http://www.dansmc.com/carbs2.htm

Jetting has nothing to do with popping on deceleration. When you decel, you're suddenly cutting fuel but your bike is still sucking air at higher RPMs. This temporarily lean condition leads to popping. Here is why:

http://www.cyclespot.com/forums/showthread.php?t=20365

Blocking off the reed valves through marbling or coaster plates is a good way to fix the problem.

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kcducttaper

Discussion Starter · #9 · Jan 23, 2015

So, it sounds like these carbs only have a LSN (low speed needle) which only adjusts the mixture at low throttle and no HSN (High Speed Needle) that takes over at mid throttle for the higher throttle mixture. That makes sense then. Thanks guys!

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PatC

· #10 · Jan 23, 2015

The pilot jet does not involve a needle per se, though the adjustment screw does have as needle-like end on it.

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